FIGS. 1A, 1B, and 1C depict conventional magnetic elements 10, 10′, and 10″ that can be used in magnetic memories. Note that recent reviews of developments in the field of magnetic memories can be found for example in: “Memories of Tomorrow”, by William Reohr et al., IEEE Circuits and Devices Magazine, pp. 17–27, September 2002; and “Magnetoresistive Random Access Memory Using Magnetic Tunnel Junctions”, by Saied Tehrani et al., Proceedings of the IEEE, vol. 91, no. 5, pp. 703–714, May 2003. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional spacer layer 16 that is conductive and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional spacer layer 16 is nonmagnetic. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. Also depicted are top contact 20 and bottom contact 22 that can be used to drive current through the conventional magnetic element 10.
The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional space layer that is an insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
The conventional magnetic element 10″ includes an AFM layer 12′, a conventional pinned layer 14″, a conventional spacer layer that is a current confined layer 16″ and a conventional free layer 18″ having a changeable magnetization 19″. The conventional current confined layer 16″ is an inhomogeneous layer mixing areas of metallic conduction (hereafter termed conductive channels 15), with high resistivity regions (hereafter termed an insulating matrix 17) that may be insulators. The conduction between the ferromagnetic layers 14″ and 18″ is essentially confined to the conductive channels 15. The conventional magnetic element 10″ is thus termed a current confined magnetoresistance effect thin film structure. The conventional magnetic element 10″ is more fully described in the context of magnetoresistive hard disk drive read-out heads in: “The Applicability of CPP-GMR Heads for Magnetic Recording”, by M. Takagishi et al., IEEE Trans. Magn. 38, 2277 (2002).
Depending upon the orientations of the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ and the conventional pinned layer 14/14′/14″, respectively, the resistance of the conventional magnetic element 10/10′/10″, respectively, changes. When the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ is parallel to the magnetization of the conventional pinned layer 14/14′/14″, the resistance of the conventional magnetic element 10/10′/10″ is low. When the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ is antiparallel to the magnetization of the conventional pinned layer 14/14′/14″, the resistance of the conventional magnetic element 10/10′/10″ is high. To sense the resistance of the conventional magnetic element 10/10′/10″, current is driven through the conventional magnetic element 10/10′/10″. Typically in memory applications, current is driven in a CPP (current perpendicular to the plane) configuration, perpendicular to the layers of conventional magnetic element 10/10′/10″ (up or down, in the z-direction as seen in FIG. 1A, 1B, or 1C). In this configuration, current is driven between the top contact 20, 20′, 20″ and the bottom contact 22, 22′, and 22″, respectively.
In order to overcome certain issues associated with magnetic memories having a higher density of memory cells, spin transfer may be utilized to switch the magnetizations 19/19′/19″ of the conventional free layers 10/10′/10″. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic elements 10 and 10″. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000), “Conductance and exchange coupling of two ferromagnets separated by a tunneling barrier”, by J. Slonczewski, Phys. Rev. B 39, 6995 (1989), and “Observation of spin-transfer switching in deep submicron-sized an low-resistance magnetic tunnel junctions” by Y. Huai et al., Appl. Phys. Lett. 84, 3118 (2004). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. This transfer of angular momentum can be considered a spin transfer torque (STT) acting on the free layer magnetization 19′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). The threshold current at which spin transfer induced switching can occur is termed the critical current, Ic. In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, it is generally believed that the conventional free layer 18′ should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18/18′/18″ of the conventional magnetic element 10/10′/10″. For example, in the conventional magnetic element 10′, the magnetization 19′ of the conventional free layer 18′ can be switched from antiparallel to the magnetization of the conventional pinned layer 14′ to parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′ when current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction.
The magnitude of the critical current, Ic, can be determined using the prevalent spin transfer spin-torque model described in J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1–L5 (1996), and further expanded in particular in: “Field dependence of magnetization reversal by spin transfer”, by J. Grollier et al., Phys. Rev. B 67, 174402 (2003). According to Slonczewski's model, the switching current density Ic for the free layer of a spin transfer stack is proportional to:αtMs[Heff−2πMs]/g(θ)                where:        α=the phenomenological Gilbert damping parameter;        t=the thickness of the free layer;        Ms=saturation magnetization of the free layer;        Heff=effective field for the free layer;        g(θ) reflects the spin-transfer efficiencyThe effective field, Heff, includes the external magnetic field, shape anisotropy fields, in-plane and out-of-plane (i.e. perpendicular) anisotropies, and dipolar and exchange fields. The perpendicular anisotropy typically arises from crystalline anisotropy. The term g(θ) depends on the relative angular orientations of the magnetizations of the conventional pinned layer 14′ and the conventional free layer 18′.        
Thus, the critical current Ic is proportional to the Gilbert damping parameter α of the conventional free layer 18′. This is believed to be equally applicable to spin transfer in conventional spin valve magnetoresistance effect element such as 10 and conventional current confined magnetoresistance effect element 10″. The Gilbert damping parameter α is a dimensionless parameter, which quantifies the level of dynamic damping experienced by the conventional free layer magnetization 18′. Assuming the remaining factors remain the same, a reduction in α results in a proportional reduction of Ic, while an increase in α results in a proportional increase of Ic. For a thin conventional magnetic free layer 18′ embedded in a multilayer structure, it has been shown that the total damping coefficient, α, can be in general broken into three contributions:α=α0+(δαout+δαin)t0/tf                where:        α0=the intrinsic damping parameter;        δαout=surface contribution originating from processes taking place at the outer interface of the free layer, for example between the conventional free layer 18′ and the top contact 20′;        δαin=surface contribution originating from processes taking place at the inner interface of the free layer, for example between the conventional free layer 18′ and the barrier layer 16′;        t0=arbitrary scaling length;        tf=thickness of the free layer expressed in nanometersThe intrinsic damping parameter α0 is dependent only on the material used to create the conventional free layer 18′. The arbitrary scaling length, t0, is conveniently taken equals to three nanometers without loss of generality. The thickness of the conventional free layer 18′, tf, is the thickness of the free layer expressed in nanometers.        
The inner surface contribution to the damping parameter, δαin, depends on the detail of the structure and composition of the interface between the conventional free layer 18′ and conventional barrier layer 16′, the conventional barrier layer 16′ itself, possibly the interface between the conventional barrier layer 16′ and the conventional pinned layer 14′, and the conventional pinned layer 14′. In particular, the magnetic element 10′ may experience a significant and detrimental contribution of δαout that can be traced back to “spin pumping” taking place at the top (outer) interface of the free layer 10′. Spin pumping damping is generated by losses of angular momentum from the time dependent magnetization of the conventional free layer 18′ by exchange coupling with the free electrons able to leave the free layer into the top contact 20′. Such effects are described in details for example in: “Dynamic stiffness of spin valves” Y. Tserkovnyak et al., Phys. Rev. B 67, 140404(R) (2003). Such spin pumping induced damping is a limiting factor in the ability to decrease Ic to desirable levels for magnetoresistance effect thin film structures as known in the prior art, with free layer thickness typically ranging from one to five nanometers.
Thus, although spin transfer functions as a mechanism for switching the conventional magnetic elements 10/10′/10″, one of ordinary skill in the art will readily recognize that a high current density is typically required to induce switching for the conventional magnetic elements 10/10′/10″. In particular, the switching current density is on the order of a few 107 A/cm2 or greater. Thus, a high write current is used to obtain the high switching current density. The high operating current leads to design problems for high density MRAM, such as heating, high power consumption, large transistor size, as well as other issues.
Accordingly, what is needed is a system and method for providing a magnetic memory element having elements that can be switched using spin transfer at a lower current density and that consume less power. The present invention addresses such a need.